Group 15 - Pnicogens

1. Group 15 - Pnicogens • N is diatomic; P, As & Sbhave multiple allotropes N & P are nonmetals As & Sb are metalloids Bi is a metal. • They form monoatomic -3 anions. • Metal can exist as cations such as: Sb3+ and Bi3+ also exist • Five valence electrons: [N.G.] ns2 np3 • Negative ΔHelectronic attraction Images from http://www.theodoregray.com/PeriodicTableDisplay/

2. Nitrogen Reactivity Nitrogen is a very stable, unreactive, diatomic molecule (N2), as it has a strong triple bond. N2 is used as an inert atmosphere for chemical reactions and for production of electronic components. Much cheaper than the most common alternative, argon. (Air is 78% N2, 21% O2 and 1% Ar) Nitrogen reacts spontaneously with Li and with the alkaline earth metals: The reaction between nitrogen and hydrogen is exothermic, however it does Not take place without a catalyst.

3. Nitrogen N is an essential element for life, ironically most organism cannot utilize elemental nitrogen (N2), instead requiring nitrogen to be “fixed” either as as a nitrate, an ammonium salt or some other nitrogen containing metabolite. Haber-Bosch process: -The major industrial method for making ammonia - N2 and H2 are combined in the presence of catalyst at a high T (400 oC) and P (~200 bar): - The reaction is forced forward removing NH3 as it is produced by cooling it to a liquid. Why? - NH3 condenses at a much higher temperature than N2 or H2. Why? - The unreacted N2 and H2 are recycled allowing for almost 100% yield over time.

4. Haber-Bosch process

5. Ammonia • - A weak base when dissolved in water: i.e only • some of the ammonia reacts with the water: NH3(aq) is often referred to as NH4OH(aq),as the equilibrium is reactant favoured, NH3(aq) is a better description. • - Ammonia reacts completely with the strong acids. • It should be kept well away from nitric acid as the product of that reaction • is highly explosive! What is this product? • Write a balanced equation for the reaction between ammonia and nitric acid. Many other explosive compounds contain nitrogen because N2(g) is such a stable molecule that reactions producing it tend to be very exothermic. e.g. azides (N3- salts), nitrates (TNT & nitrocellulose)

6. Phosphorus • There are two major allotropes of phosphorus: • i) White phosphorus (P4) is soft and waxy and so reactive with air that it is stored under water. It can ignite spontaneously in air • ii) Red phosphorus (P, a network solid) is formed when white phosphorus is heated to 200 - 400 oC away from air. • iii) Black phosphorus and violet phosphorus also exist. They are both network solids.

7. Phosphorus • As we saw in the halogens section, phosphorus reacts with the halogens • to make PX3 which, on reaction with more halogen, makes PX5. It isn’t surprising that P4 is so reactive given its structure. Compare the bond angles in P4 to the bond angles the atoms “should” have.

8. Phosphorus • Phosphorus forms strong bonds with oxygen, and there are several • phosphorus oxides, phosphates, phosphites and phosphoric acids. • We saw that the sulfur oxides were acidic. This can be generalized to all • nonmetal oxides. The two main oxides of phosphorus are P4O6 and P4O10. • What is formed when each reacts with water? Do you remember what was unusual about phosphorous acid?

9. Phosphorus • Phosphoric acid can polymerize. • H3PO4 is “orthophosphoric acid” • ii) Two H3PO4 combine as “diphosphoric acid” (or “pyrophosphoric acid” or phosphoester) releasing a molecule of water: • ii) When a third molecule of H3PO4 is added to the chain, “triphosphoric • acid” (or phosphodiester) is produced: • The phosphodiester bond contains a • lot of easily accessed energy for • biological reactions. • Molecules such as ATP and • DNA phosphoester bonds.